For accurate evaluation of a sound field, it is important to determine where the sound comes from, namely, to get information on the arrival direction of the sound. For measurement of an indoor sound field, the proximity four-point method (cf. Kenji Endoh, Yoshio Yamazaki, Tsuyoshi Itoh “Determination and Development of Space Information by the Proximity Four-Point Method”—Architectural Acoustics Committee, Document AA85-21 (July, 1985) and Japanese Patent Laid Open No. 2000-354290)), tetrahedron vertex method or the like has been used in the past to extract sound-direction information on the basis of a difference between a time when a sound wave is incident upon one of microphones on different channels and time when the sound wave is incident upon another microphone.
In case a plurality of sound waves is incident upon one microphone at the same time, however, it is difficult to differentiate between the sound waves and make accurate estimation of the arrival directions of them. Also, since the arrival directions of the sound waves are estimated by analyzing the response of a test signal (impulse), namely, impulse response, by post-processing, it is difficult to get any real-time directional information.